The present invention relates to injection molding methods and apparatus, and, more particularly, a sequential fill valve gated injection molding system for molding reinforced thermoplastics particularly suited for producing large, thin molded components.
Recently, there has been an increase in the demand and applications for large molded plastic parts. As a result, some of these parts have become quite complex. One example of this can be seen in bumper fascia for automobiles. Design engineers are now integrating many features into the fascia such as grilles and light openings to reduce tooling and manufacturing costs. Also, to save material, fascia are designed with thinner walls. Due to the complex cavity geometries and increased flow length versus wall thickness ratios, it is often difficult to predict the actual flow pattern that will take place during mold filling. Although design software may be used to help determine the most optimum processing conditions, gate locations, and hot runner diameters for a balanced fill, quite often the expected fill pattern is not realized in practice as a result of variables such as steel dimension variations, mold temperature variations, and venting inadequacies, for example. Process engineers are therefore faced with a nonuniform fill which under certain conditions may result in decreased dimensional stability of the fascia, as well as deficiencies in paint adhesion characteristics and/or other surface appearance concerns.
Improved processing techniques that provide more control over the filling of large complex cavity geometries are required to meet the increased demands presented by more modem molding standards. To improve part quality, melt front advancement must be further controlled during the actual filling phase to achieve a more uniform filling and packing distribution. In addition, there is a continuing interest in pursuing further time and cost efficiencies associated with part manufacture.
U.S. Pat. No. 5,762,855 discloses an injection molding system for large molded components that may be used to enhance the quality of the final molded part in a timely and cost-efficient manner. Specifically, that patent discloses a method for molding large components in a mold having at least one mold cavity. Plasticized material is introduced into a cavity mold through a manifold. The manifold has at least two spaced valve gates that are independently opened and closed as directed by a controller to selectively communicate plasticized material from the manifold to the mold cavity at separate locations in the mold cavity. The controller directs the valve gates to sequentially open and close during the filling phase so as to achieve the desired melt front advancement within the mold cavity. Once the mold cavity has been filled, the valve gates are closed to effectively seal the manifold from the mold cavity. The closed valve gates thereby assist in allowing the plasticized material within the manifold to be held in compression while the mold cavity is open for removal of the molded component from the mold cavity, so as to prevent appreciable expansion of the material that has been found to result in imperfections, such as splay, in molded products.
While the invention disclosed in the ""855 patent is particularly useful for producing large, thin walled plastic parts, its usefulness is limited by the structural characteristics of the plastic material conventionally used. That is, while the invention disclosed is particularly suited for parts with large planar dimensions and thin walls, the usefulness of the disclosed invention is limited by the fact that the parts produced can be only so large or so thin before the parts lose their structural integrity and impact resistance.
Heretofore, in order to reinforce various thin plastic parts such as fascia, such parts would conventionally be reinforced by mineral fillers or glass fibers. However, such reinforcement has a deteriorating effect on impact resistance of the part. Moreover, the conventional reinforcement materials are inadequate to enable the full benefits that might otherwise be achieved by the methodology disclosed by the ""855 patent.
The disadvantages of the prior art may be overcome by providing a method for molding large, thin components in a mold having at least one mold cavity. Reinforced plastic pellets are provided, which pellets comprise at least one thermoplastic material and reinforcement particles dispersed within the at least one thermoplastic material, the reinforcement particles comprise less than 15% of a total volume of the pellets, and at least 40% of the reinforcement particles have a thickness of less than about 50 nanometers. The reinforced plastic pellets are melted to produce plasticized material therefrom. The plasticized material is communicated through a manifold to a cavity mold. The manifold has at least two spaced valve gates that are independently opened and closed as directed by a controller to selectively communicate the plasticized material from the manifold to the mold cavity at separate locations in the mold. A primary injection pressure is applied to the plasticized material in the manifold to fill the mold cavity through sequential opening and closing of the valve gates as directed by the controller. A secondary injection pressure is applied to the plasticized material in the manifold to continue to fill the mold cavity. The secondary injection pressure is less than the primary injection pressure. The valve gates are closed to seal the manifold from the mold cavity when the mold cavity is filled. The plasticized material is held within the manifold in compression while the mold cavity is open for removal of the molded component from the mold cavity. The compression is maintained with the assistance of the closed valve gates to prevent appreciable expansion of the material.
Other objects and advantages of the present invention will become apparent from the following detailed description.